The present invention relates to a plasma processing method, and in particular, to a plasma processing method for performing surface processing including, for example, etch processing and film deposition processing, on a semiconductor integrated circuit element, employing helicon wave excited plasma.
Keeping in mind the fineness and the sophistication of feature sizes of semiconductor integrated circuit elements, such as the size of a contact hole, provided by recent plasma processing techniques, it has been necessary to perform surface processing, in particular plasma etching, at a low pressure. The surface processing at a lower pressure gives rise to the problem of reducing the processing rate, such as etch rate and deposition rate, because the densities of ions and neutral active species are reduced at the lower pressure. Such reduction in processing rate is compensated for by using high-density plasma such as helicon wave excited plasma. Conventional helicon wave excited plasma methods are disclosed in U.S. Pat. Nos. 4,990,229, 5,091,049, and 5,122,251.
Conventional plasma processing methods employing helicon wave excited plasma have the advantage of obtaining a high degree of process gas dissociation even at low pressure. The high degree of process gas dissociation, however, gives rise to various problems. For example, in a case where the helicon wave excited plasma is applied to etch processing of silicon oxide film, the dissociation of hydrocarbon fluoride (CxHyFz) gas used as an etching gas proceeds excessively, thereby producing a significant number of fluorine atom radicals. The fluorine atom radicals are etchants for silicon (Si) substrates, as well as silicon oxide films. The silicon substrates are used under the silicon oxide film as a base material. Through the silicon oxide film, the underlying silicon substrate is also etched by the fluorine atom radicals. Since the underlying silicon is etched by the fluorine atom radicals, a high etch-selective ratio of the silicon oxide film to the underlying silicon substrate cannot be achieved.
A specific example of the aforementioned problems will be given with reference to FIG. 5. FIG. 5 is a graph relating to the dependency of silicon oxide film and silicon etch rates, as well as the etch-selective ratio of silicon oxide film to silicon (silicon oxide film etch rate/silicon etch rate) to contact hole diameter at a source power of 1,750 W. This graph was obtained from etch rate data measurements taken from a plasma etching apparatus in which a helicon wave plasma source was built, a source power of 1,750 W was applied to the helicon wave plasma source, helicon wave excited plasma in C4F8+H2 etching gas under a pressure 1.33 Pa was generated, and a silicon oxide layer formed on the silicon substrate was etched.
In general, a high power source is applied into a helicon wave plasma source to generate a high density plasma. In this evaluation, the applied source power is as high as 1,750 W.
In the coordinate system of the graph of FIG. 5, the horizontal axis indicates the contact hole diameter in xcexcm, the left vertical axis indicates the etch rates of the silicon oxide film and underlying silicon (xc3x85/min), and the right vertical axis shows the etch-selective ratio of silicon oxide film to underlying silicon. Because the silicon oxide film is laid on the underlying silicon substrate, the etching of the silicon substrate is started at the completion of the etching of the silicon oxide film. The thickness of the silicon oxide film is 1 xcexcm. The etch rates of the silicon oxide film and the silicon substrate depend on the diameter of the contact hole. Referring to the FIG. 5, curve 51 represents the change in silicon etch rate, curve 52 represents the change in etch-selective ratio, and curve 53 represents the change in silicon oxide film etch rate.
From FIG. 5, it is clear from curve 52 that the etch-selective ratio of the silicon oxide film to the underlying silicon decreases with respect to the contact hole diameter. When the contact hole is, for example, 0.5 xcexcm in diameter, the selective ratio of the silicon oxide film to underlying silicon is at most 23. This shows that the conventional plasma etching method employing helicon wave excited plasma has a problem of an extremely low etch-selective ratio of the silicon oxide film to underlying silicon.
To overcome the above-described problems, an object of the present invention is to provide a plasma processing method for controlling the degree of dissociation of a process gas in a helicon wave excited plasma. Another object of the present invention is to provide a plasma processing method for achieving a high etch-selective ratio of a silicon oxide film to underlying silicon.
In a first embodiment of the invention, there is provided a plasma processing method comprising applying a source power to a plasma generator through an antenna of a helicon wave plasma source system for generating a helicon wave excited plasma. The applied source power is set lower than a source power corresponding to a discontinuous change, what is called a mode jump, on a characteristic line of an electron density or a saturated ion current density as a function of a source power. The helicon wave excited plasma generated by the applied source power set lower than the source power corresponding to the discontinuous change is used to perform required surface processing of a substrate.
The first embodiment of the invention features setting the applied source power lower than a source power corresponding to a discontinuity on a characteristic line of an electron density as a function of source power (in the graph showing dependency of electron density on source power) or a saturated ion current density as a function of a source power (in the graph showing dependency of saturated ion current density on source power). Setting the applied source power lower than a source power corresponding to the discontinuous change on the characteristic line controls a degree of dissociation of the process gas. The control of the degree of dissociation prevents individual atoms constituting molecules of process gas from being liberated.
In contrast to this, setting the applied source power higher than a source power corresponding to a discontinuous change on a characteristic line causes excessive dissociation of the process gas, so that a considerable number of atom radicals are generated. When silicon oxide film is etched by hydrocarbon fluoride (CxHyFz) gas, as etching gas, in which helicon wave excited plasma is generated, the generated fluorine atom radicals etch the underlying silicon, and thereby, reduces the etch-selective ratio of the silicon oxide film to underlying silicon.
However, if the applied source power is set lower than a source power corresponding to a discontinuous change of a characteristic line, the setting of the applied source power controls the dissociation to liberate the fluorine atom radicals from the hydrocarbon fluoride molecules, so that a high etch-selective ratio of silicon oxide film to underlying silicon can be achieved in etch processing of the silicon oxide film.
In the plasma processing method of a second embodiment of the present invention, a source power is applied to a plasma generator through a helicon wave plasma source antenna. The applied source power is set lower than a source power corresponding to a discontinuous change in a gradient of a straight line approximately linearized to a characteristic line of an electron density or a saturated ion current density as a function of a source power. Using the helicon wave excited plasma generated by the applied source power set lower than the source power corresponding to the discontinuous change in the gradient, the required surface processing to the substrate is performed.
The second embodiment of the invention features setting the applied source power lower than a source power corresponding to a discontinuous change in a gradient of a straight line approximately linearized to a characteristic line of an electron density or a saturated ion current density as a function of a source power. In the second embodiment of the invention, the discontinuous change in the gradient of the straight line approximately linearized to the characteristic line is taken as an index causing an excessive dissociation of the process gas.
An applied source power higher than a source power corresponding to a discontinuous change in the gradient causes excessive dissociation of the process gas. As a result, an applied source power higher than a source power corresponding to a discontinuous change in the gradient of a straight line approximately linearized to a characteristic line produces a considerable number of constituent atom radicals of the process gas molecules. Therefore, to control the degree of dissociation of the process gas, the source power, considered as an index to which a discontinuous change in the gradient of a straight line approximately linearized to a characteristic line, is set lower than a source power corresponding to this discontinuous change.
In one form of the invention, the plasma processing method used in the first or second embodiment of the invention is a plasma etch processing. Thus, the plasma etch processing is the most suitable for a plasma processing method of the invention.
In another form of the invention, the plasma processing method used in the aforementioned form of the invention is performed on silicon oxide film.